Wave length modulated wave signaling



March 29, 1949- c. w. HANSELL WAVE LENGTH MODULATED WAVE SIGNLING i Sheets-Sheet l Original Filed Nov. 27, 1956 ATTORNEY March 29, 1949. c. w. HANSELL 2,465,448

WAVE LENGTH MODULATED WAVE SIGNALING Original Filed Nov. 27, 1956 4 Sheets-Sheet 2 Jay. Q

LLOWAB/.E FREQUNCY DEV/T/U/V /A/ KILOCYCLES N (u ih N 0 l0 MOULAT//VG FREQUENCY /N KILOCYCLES L I 0.067 HENRY mug xL =2fo0nAT 5000 crm-s =4f00n Ar fo, 00o crcLEs lNvENToR C. W. HANSELL BY MM ATTORNEY March 29, 1949.

C. W. HANSELL,

WAVE LENGTH MODULATED WAVE SGNALING 4 Sheets-Sheet 3 V l mm/im' waz/Arm g ,due/0 WAVE AMPL/F/m AND g mmm/cr afnam/470k 0F mur. a camper/N6 0R Erika/m15 TYPE f 1 o wirr/URK INVENTOR C. W. HAgSELL 'EY 754g ,gf/WL,

ATTORNEY March 29, 1949- c;. w. HANSELL, 465448 WAVE LENGTH MQDULATED WAVE SIGNLNG Original Filed Nov. 27, 1936 4 .Sheets-Sheet 4 F13 y. s

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Osa'lar C, INVENTOR fnegueacg ATTORNEY Patented Mar. 29, 1.949

WAVE LENGTH MODULATED WAVE SIGNALING Slarence W. Hansell, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Original application November 27, 193.6, Serial No.

112,959, now Patent No. 2,179,182, dated November 7, 1939. Divided and this application October 4, 1939, Serial No. 297,777

1 claim. (o1. 25e- 6) This application is a division of my United States application #112,959, filed November 2". 1936, now Patent #2,179,182, dated November 7, 1939. In operating a frequency modulation communication circuit it is possible to eliminate a considerable amount of the noise that would be heard in an equivalent amplitude modulation system. This is because a balanced frequency modulation receiver, particularly one with amplitude limiting of the energy input to its frequency modulation detector, tends to suppress the effect of all incoming noise in side frequencies near the frequency of the carrier wave. Noise frequency components which would normally give low audio frequency beats with the carrier tend to balance out in the output of the balanced frequency modulation receiver. The balance is substantially perfect for those noise components which would give nearly zero frequency audio output from the receiver but the balance decreases to nearly zero for noise components lying at the extreme limits of the spectrum for which the receiver is designed. As a result, a frequency modulation receiver of the balanced type designed to accommodate zero to 10,000 cycles modulation has its maximum noise output at 10,000 cycles, but at 5,0 cycles the noise is reduced to half amplitude. At 1,000 cycles the noise is reduced to at 100 cycles to 1%, etc., down to zero at zero frequency.

Frequency modulation receivers of the balanced type referred to above are known in the art and examples of such receivers have been shown and claimed in Usselman Patent #1,794,932, dated March 3, 1931, and in Crosby application #45,409, filed October 17, 1935, Patent #2,071,113, dated February'l, 1937. Other balanced frequency modulation receivers, in addition to those mentioned above, are known in the art. See, for example, my Patents #1,867,567, #1,922,290 and #1,938,657.

The balanced frequency modulation receiver also discriminates against useful side frequencies produced at the transmitter by the lower frequency modulations which lie near the carrier. However, this discrimination is balanced out by the greater number and spectrum width of side frequencies produced by low frequency modulation compared to that which would be obtained in the amplitude modulation system. The nal result is an average gain, due to the balancing, of about 3 to 1 in power with respect to signal-to-noise ratio which would be obtained in receiving wave energy from an amplitude modulated transmitter of equal carrier power. There is an addi- Y 2 tional gain of about 5 to 1 due to the possibility of increasing the output from a given transmitter when we change from amplitude to frequency modulation. Thus, using frequency modulation instead of amplitude modulation and maintaining substantially equivalent conditions, may make .an improvement in signal-to-noise power ratio as high as 15 to 1.

It is known in the radio art that a still greater improvement in signal-to-noise ratio may be obtained by increasing the frequency deviation at the transmitter in response to the modulating potentials, while, at the same time, making correspondingly wider frequency pass band adjustments at the receiver. In fact, so long as the signal is considerably stronger than the noise, the signal to noise power ratio will be improved about in proportion to the square of frequency band used.

My present invention is an improvement on the methods and means for frequency modulation and frequency demodulation by balanced receivers known in the art and on the wide frequency deviation systems known in the art. The latter systems require a transmitter to occupy greater space in the ether and so will lead to an increase in interference or a reduction in the number of transmitters which may be operated without mutual interference. 'My present invention may be applied without widening the frequency band required for a transmitter or it may be used in combination with moderate widening of the band. In effect, it permits one to use the minimum frequency spectrum required by the modulation or a chosen band of greater width, more effectively than has been heretofore possible and in a manner to increase the signal-to-noise ratio.

In describing in detail the outstanding features of my novel method of and means for signalling by frequency modulated wave energy, reference will be made to the attached drawings. In the drawings:

Figure 1 is a chart showing the relative amplitudes of a carrier and side frequencies produced by phase modulation of the carrier;

Figure 2 is a curve illustrating the manner in which the relative amplitudes of modulating potentials at various frequencies may be distorted to obtain the operating characteristics desired in the frequency modulator;

Figure 2a is a curve illustrating the manner in which the audio frequencies at the receiver must be distorted to obtain an overall linear output;

Figures 3, 4, and 5 illustrate audio frequency potential correcting or distorting networks;

Figure 6 illustrates diagrammatically the essential elements of a frequency modulator which includes an audio frequency potential correcting network such as those shown in Figure 3; while Figure 7 illustrates diagrammatically by rectangles the essential elements of a frequency modulation receiver including an audio frequency potential correcting network such as illustrated in Figures 4 and 5.

Figs. 8 and 10 show preferredforms of frequency modulated wave demodulators to be used say in a receiver as illustrated in Fig. 7 ahead of the audio-frequency correcting network.

Figs. 9 and 11 are curves of the characteristics of the receivers of Figs. 8 and 10, respectively.

Figure 1 of the drawings shows the relative amplitudes of carrier and side frequencies produced when the carrier is modulated in phase. In this drawing the ordinates indicate the relative amplitudes of the carrier and various side frequencies. The abscissa indicates phase deviation of the carrier and side frequencies in radians.

yThe carrier has been designated Ja (X), while the side frequencies produced by any one modulating frequency have been indicated by J1 (X), J2 (X), Ja (X), etc., in the order of their remoteness from the carrier. This chart also shows the relative amplitudes of a carrier and side frequencies of a frequency modulated wave, if we remember that the scale marked Argument of the function=X is the phase deviation of the carrier in radians, which we may call 4). Also, the angle =D/f where f is any one modulating frequency and D is the frequency deviation of the carrier wave produced by the energy of frequency f.

For further information concerning the analysis of frequency modulated waves into component frequencies, reference may be made to a paper in Proceedings of the Institute of Radio Engineers, by Bolth Van Der Pol, entitled Frequency modulation. This paper appears in vol. 18, No. 7, July, 1930. See, also, a paper by Hans Roder entitled Amplitude, phase and frequency modulation, in Proceedings of the Institute of Radio Engineers, vol. 19, No. 12, December 1931. Each of the foregoing papers lists additional references.

If we have a frequency modulated transmitter and intended to handle modulating frequencies from 0 to 10,000 cycles, and we wish to specify that no one modulating frequency, given full modulation, can produce side frequencies outside the normal band exceeding 1% in voltage of the carrier wave, we can see immediately from the curves in Figure 1 that at 5,000 cycles the phase deviation must be,limited to 0.3 radian, because the second order side frequencies (of amplitude J2 (Xl) produced by` the 5,000 cycles modulating frequency reaches 1% and falls at the limit of the permissible band, i. e., plus and minus 10,000 cycles. This limitation on phase deviation corresponds to a limitation of peak frequency deviation of D=5,000 0.3 or 1,500 cycles for a modulating frequency of 5,000 cycles.

An equal modulating voltage at 1,000 cycles will also produce 1,500 cycles frequency deviation in the usual frequency modulator and the corresponding phase deviation will be d =1509fmoc or 1.5 radians. However, in the case of 1,000 cycles modulation we need not be concerned about exceeding our permissible plus and minus 10,000 cycle band until the tenth order side frequency Jio (X) is reached and by inspection of Figure 1 we see that Jm (X) reaches an amplitude of 1% of the carrier for a phase deviation of about 6.2 radians corresponding to a frequency deviation of =F==6.2 1000=6200 cycles. Therefore, the 1,000 cycle modulation will not produce out of band side frequencies exceeding 1% up to a frequency deviation of about plus and minus 6200 cycles. From this it will be seen that in the ordinary frequency modulator, we are not using all the deviation permissible. i. e., plus or minus 6200 cycles instead of plus and minus 1,500 cycles.

If we again refer to the chart of Figure 1 we will see that the 10th or higher order side frequencies do not exceed 1% of the carrier amplitude in case =the pink deviation is equal to or less than 6.2 radans. Thus we may increase the phase deviation at 1000 cycles up to 6.2 radians, or more than4 to 1, as compared to the deviation produced at 1,000 cycles by modulating potentials of the same amplitude as the permissible limit at 5,000 cycles before we exceed our allowable 1% for any side frequency outside the band of plus and minus 10,000 cycles.

At all modulation frequencies between 5000 and 10,000 cycles all but the first order of -side frequencies will fall outside the permissible band. that is, carrier plus and minus 10,000 cycles. If we limit out of band frequency amplitudes to 1% of the carrier power` then the phase deviation, may be allowed to reach but must not exceed .3 radian for any modulating frequency in the upper half of the modulation band. The permissible frequency swing or frequency deviation in the 5,000 to 10,000 cycle band may, therefore, be D=F=0.3F and increases from 1500 to 3000 cycles as the modulation frequency increases from 5000 to 10,000 cycles. That is, the permissible frequency swing for modulating potentials between 5000 and 10,000 cycles is proportional to the modulation frequency and if the modulation frequency potentials are increased in proportion to the modulation frequency from 5000 to 10,000 cycles, the desired correction is obtained. This is illustrated in Figure 2 of the drawings where the curve slopes upward in a positive direction substantially linearly from 5000 cycles to 10,000 cycles.

At a modulating frequency of 10 cycles, we may produce a frequency deviation very close to plus and minus 10,000 cycles, whereas normally our deviation would be only about plus and minus 1,500 cycles, the same as at 5000 cycles. This is because, as may be found by inspecting a table of Bessell functions representing the relative amplitudes of carrier and side frequencies, only phase deviations above 10000/10 produce frequencies which fall outside the permissible band and they are so weak that they can be entirely disregarded if the phase swing does not exceed the value of 10000/10. But a phase swing of 10009@ gives a frequency deviation=10 10009i0= 10,000 cycles as mentioned above. Similar reasoning shows that a frequency swing of approximately 10,000 cycles is allowable for all extremely low modulating frequencies.

At 2500 cycles modulating frequency only the 4th order of side frequencies fall at the limit oi."

the permissible band of plus and minus 10,000 cycles and to maintain the amplitudes of these side frequencies below 1% of the carrier we may have a peak deviation of about =1.5 radians which gives a permissible frequency swing of 2500 1.5 or 3750 cycles.

From this it will be seen that in order to use our whole permissible frequency band for each modulating frequency We should increase the amplitude of the lower modulating frequencies with respect to the amplitude of the mean frequency. This has been illustrated in Figure 2 of the drawings. It will be noted here, however, that the amplitude of the modulating potentials as they leave 5000 cycles and decrease towards zero cycles should not be increased linearly. At nearly zero modulating frequency We may have a peak frequency deviation of plus and minus 10,000 cycles, at 1000 cycles modulation we may have a peak frequency deviation of 6000 cycles, at 2500 cycles we have a peak frequency deviation of 3750 cycles, at 5000 cycles modulation we may have a peak frequency deviation of .1500 cycles and for higher modulating frequencies up to 10,000 cycles, we may increase the frequency deviation linearly up to 3000 cycles again.

The curve in Figure 2 shows approximately the way` in which I may distort the frequency characteristics of the transmitter in the assumed case without exceeding 1% amplitude on any side frequency more than 10,000 cycles removed from the carrier. Obviously, preferably, I distort the the frequency characteristics of the transmitter in this way in order to maintain a better signalto-noise ratio at the receiver. I may then distort the receiver response characteristi-c oppositely to the frequency distortion in the transmitter, as illustrated in Fig. 2a, and thus end up with a correct reproduction of the audio frequency modulation at the output of the receiver with a greatly reduced noise level. In the assumed case, I have shown that the improvement in signal-to-noise ratio amounts to about 17 to 1 in power at 1000 cycles, where the ear is quite sensitive, compared to the ratio obtainable with ordinary frequency modulation. The improvement at various frequencies will be about as follows:

The mean or overall improvement in signal-tonoise ratio in the foregoing example will depend upon the normal frequency distribution of modulation energy and of noise but will tend to range above and below about to 1 in power. If noise is predominantly low in frequency the overall gain may range up to about 45 to l.

If I distort the frequency response characteristic of a transmitter in accordance with the invention to take full advantage of the permissible maximum frequency deviations at each modulating frequency and also take advantage of the proposal to add additional frequency response distortion in accordance with the inverse of the peak values of modulating voltages at different modulating frequencies then I may expect to obtain an overall gain in signal-to-noise ratio ranging up to perhaps 100 to 1 in power.

It should be noted that, in addition to reducing ordinary noise the scheme brings about a great relief from the effects of hum in transmitters and receivers. Practically all transmitters, for example, have the power potentials supplied to the electrodes of the vacuum tubes from rectifiers fed with 50 or 60 cycles A. C. current.

As a result, we have had much diiiiculty in eliminating hum at say, 60, 120, 180, 240, 360, and 720 cycles. Of these, hum at 120 cycles has been most troublesome. The present scheme, in the assumed case, would reduce the effects of 120 cycle frequency modulation hum by about 37 to 1 in power.

Of course, the fundamental reason for the possibility of distorting the frequency characteristic of the transmitter on frequencies below the midfrequency lies in the decreasing spacing between side frequencies as the modulating frequency is decreased. The possibility of distorting the characteristic above the mid-frequency arises from the setting of a definite limit on side frequency energy outside the prescribed band and the discontinuous way in which the limit is approached.

If the various modulating frequencies to be transmitted are not of equal maximum amplitudes, I may modify the frequency distortion response characteristics of Figure 2 so that the maximum value of any one modulating frequency component will reach the maximum allowable frequency deviation. Speech, for example, usually has maximum peak values at about 800 cycles and progressively lower value at higher and lower values. I contemplate also modifying the relative amplitudes of modulation inputs to the transmitter to t the frequency distribution of peak amplitudes in the modulation in order to obtain maximum permissible frequency deviation for each modulating frequency.

In practice we may prefer to leave the frequency characteristic undistorted in the upper half of the frequency band but to use distortion in the lower half very roughly in the proportions shown in Figure 2.

Obviously, the example I have used to illustrate my invention is not the only condition which might be assumed or. met with in practice. I might have assumed that conditions permitted as much as 5% or 10% peak amplitudes of side frequencies outside the transmitter band. Then the values of permissible frequency deviation at various modulating frequencies would have been different but could readi'y be determined with the 'aid of Figure 1. Also, I might have assumed that a transmitter` to handle modulating frequencies of 0 to 10,000 cycles could be allowed to occupy a normal band width of plus and minus 20,000 cycles, 50.000 cycles, 100,000 cycles, etc. Then, by setting a limit on out of band side band strength, the permissible distortion in frequency response, of the character illustrated in Figures 2 and 2a, could readily be determined, with the aid of Figure 1, in the same manner as in the example given.

I have not attempted to work out frequency distortingr networks for carrying out my invention with nearest possible perfection since there already exists a very well known art on this subject in connection with wire line telephone practice. However, as a rough approximation of one type of frequency distorting circuit for use at the transmitter, I have drawn a schematic diagram of a circuit in Figure 3 and indicated relative values of the component parts very approxi-` mately.

Figures 4 and 5 represent types of circuits which may be used at the receiver to counterbalance the distortion introduced in the transmitter frequency characteristic. In practice somewhat more complicated circuits may be used to obtain more exactly the characteristics required just as more complicated circuits are arrived at and used in the well known wireline telephone art in correcting the frequency characteristics of lines and apparatus. It is my belief that one skilled in the art, having been instructed in respect to the frequency distorting characteristics required; having been shown simple examples of suitable distorting networks, and having been told to consult the voluminous telephone wire line art on similar devices, will require only ordinary diligence and persistence in approaching as nearly as necessary to the ideal performance of my invention. The limitations of circuits for application of my invention will make it substantially impossible to obtain exactly the frequency distortion characteristics illustrated in Figure 2 and Figure 2a but exact matching of the theoretically optimum characteristic is not necessary in order to obtain the greater part of the advantages offered by my invention.

The method and means of the present invention may be applied to many phase and frequency modulation systems known in the art today. For example, I may use a frequency or phase modulator as illustrated in any of the many United States patents disclosing frequency or phase modulators or as disclosed in:

Hansell, United States application #681,945, July 24, 1933, Patent #2,121,737, issued June 21, 1938.

Chireix, United States application #585,489, January 8, 1932, Patent #2,076,264, dated April 6, 1937.

Crosby, United StatesI application #588,309, January 23, 1932, Patent #2,081,577, dated May 25, 1937.

Lindenblad, United States application #13,886,

March 30, 1935.

Hansell, Patent #2,027,975, dated January 14,

Hansell, Patent #1,830,166, dated November 3,

Hansell, Patent #1,819,508, dated August 18, 1931.

Hansell, Patent #1,803,504, dated May 5, 1931.

Hansell, Patent #1,787,979, dated January 6, 1931.

'I'he limitations of circuits for the application of my invention makes it desirable or necessary to use a distortion characteristic somewhat modifled from that shown in Figure 2 but my scheme may still be applied and combined with frequency modulators of any type including sideband frequency modulators or over-modulated frequency modulators or phase modulators such as, for example, disclosed in the Chireix and Crosby patents mentioned above. By "frequency modulation as employed in this specification, I mean modulating a wave, other than its amplitude in accordance with signals to be transmitted.

In describing my invention, I have given an example in which the output amplitude of any one frequency lying outside the minimum transmitter band is limited to 1% of the carrier amplitude. Although this 1% value is a good practical value to set as a limit, there may be many cases in which a lower or a higher limit are desirable. If we change the limit then the optimum frequency distortion characteristic illustrated in Figure 2 will be modified. I have shown how the characteristic of Figure 2 was arrived at so that, with a table of Bessel functions or curves such as Figure 2, anyone practicing this invention may readily obtain optimum distortion characteristic curves for any assumed limit. For additional information concerning Bessel functionssee the book, Bessel Functions for Engineers, by N. W.

McLachlan, published by the Oxford University Press in 1934 and the references cited in the back of the book.

The distorting network may be of any suitable type and, for example, may be as illustrated in Figure 3. If the modulator is one in which the frequency change is not proportional to instantaneous modulation input potential regardless of modulation frequency then obviously the distorting network characteristics will be modified to obtain the desired maximum allowable modulation at each modulating frequency.

Any 'type of frequency modulated receiver known in the art today may be used. Preferably. I use a. demodulator of the balanced type as illustrated in Usselman Patent #1,794,932 or in Crosby application #45,409, filed October 17, 1935, Patent #2,017,113, dated February 18, 1937. The essential features of a demodulator are illustrated in Figure 7 wherein wave energy pick-up means such as a line or an antenna, feeds a receiving, amplifying, and demodulating means of either the radio-frequency or heterodyne type. The output of the demodulating means is connected by a correction circuit of any type such as, for example, the type illustrated in Figures 4 and 5 to the indicating means.

In Figs. 8 and 10, I have shown demodulators of a preferred type. Fig. 8 corresponds to Fig. 1 of Usselman Patent 1,794,932, while Fig. 10 corresponds to Fig. 1 of Crosby Patent 2,071,113. Fig. 9 corresponds to Fig. 2 of the Usselman patent and is a curve illustrating the demodulators characteristic. Fig. 11 corresponds to Fig. 2 of the Crosby patent and is a curve illustrating the characteristic of the demodulator of Fig. 10. Referring to Fig. 8, frequency modulated energy is supplied by means of the conductors 2 to the resonant analyzer circuits 4 and 6, by way of coupling transformers 8 and I0. Potentials of opposite phase are supplied from the circuits I and 6 to the control electrodes of the detector tubes I2 and I4. The anodes of the tubes are connected in series by an output transformer I 6, which is by-passed for radio frequencies by a condenser I8. As so far described this circuit is slightly similar to many symmetrical circuits now known, but it is importantly different in the use of independent input circuits 4 and 6 which are differently tuned.

The operation of this circuit may be more readily explained in connection with Fig. 9, in which curve 40 is a resonance curve for the circuit 6. These curves have maxima for frequenoies FI and F2 respectively, which lie either side of the mean frequency Fm. The operating frequency range may be represented by a shaded area, as shown.

Assume that the instantaneous frequency is Fm. In such case the circuits I and 6 will be excited to equal potentials, and the outputs in the primary of the transformer I6 will neutralize one another. This will be the zero potential point of the low frequency output. As the frequency swings toward the lower frequency FI the lower frequency circuit 4 will be more strongly energized, and the higher frequency circuit 6 more weakly energized, in consequence of which there will be a current ow in the transformer I6. As the frequency again reaches the mean frequency Fm alternating current flow in the transformer I6 again will pass through zero. When the frequency increases towards F2 the higher frequency circuit 6 will be more strongly energized, and the lower frequency circuit 4 more weakly energized. and there will be a resultant current now through the transformer Il in the opposite direction. On the other hand, amplitude variations, while causing similar variations in the outputs of each of the analyzer circuits, do not affect the resultant output because they are combined in opposition. Because each half of the resonance characteristic consists of a similar portion of each of two resonance curves the resonance characteristic is symmetrical, and because the effective or combined characteristic is the resultant of two differently curved characteristics the resultant is more nearly straight, and a wider frequency band may be utilized if desired. In an exactly similar way the resultant effective rectification characteristic is made more symmetrical, and by reason of an inherent property of'symmetrical circuits numerous harmonics are suppressed.

It can be seen from an inspection of Fig. 9 that because the curvature of the combined halves of the working portions of the resonance curves are curved in opposite sense that the resultant curve will be more nearly straight, and that by the use of two similar resonance curves the resultant curve may be made symmetrical even if not perfectly straight. Aside from the advantageous properties of the balanced analyzer circuits it is to be appreciated that all of the advantages concomitant with the use of balanced tube circuits will be obtained, for example, a symmetrical resultant characteristic, the elimination of harmonies, and the withdrawal from the direct current source of practically constant current, thereby keeping the applied potential more nearly uniform.

In Fig. l10, the circuits Il! and Ill are pref erably tuned to resonance at frequencies below and above the lowest and highest frequencies, respectively, of the wave of intermediate frequency supplied to the control grids of tubes ill and lil. When these circuits are tuned, as

stated, the parallel circuit H6 is tuned to re-- sonance at a frequency above the highest intermediate frequency modulation frequency, while the parallel tuned circuit Il. is tuned to resonance at a frequency lower than the lowest frequency of the intermediate frequency energy. When so tuned, the circuits lll, ill and lil, III will have frequency response characteristics as illustrated by the characteristic curves A and B. respectively, of Fig. 1l.

In operation, energy of an intermediate frequency is fed to the two coupling tubes Ill and Ill having the conversion filters lll, llt and III, Ill in their plate circuits for converting the frequency modulation into amplitude modulation. The carrier frequency FC is caused to fail at the middle of the linear portions of the sloping characteristics ofA the tuned circuits connected as shown. By adjusting the timed circuits so that outside of the intermediate frequency channel, only the linear portions of the characteristics are utilized. The outputs of the nlters H2, IIS and IM, Il! are fed to the detectors |20 and |22 of Fig. 10, whose audio outputs are combined with the transformer T in push-pull or parallel cornbination, depending upon the switch RS. With the switch in the push-pull position, frequency modulation may be received and amplitude modulation balanced out. With the switch in the push-pull position, second harmonic square law detector distortion is completely balanced out. With the switch in the parallel position, amplitude modulation may be received and frequency modulation balanced out.

What is claimed is:

A radio receiver. for receiving and translating a radio wave so modulated in frequency that the relation of the frequency deviations of the modulated radio wave to the amplitudes of signalling waves is greater for certain higher frequencies of the signalling waves than for certain other relatively lower frequencies of the signalling waves, comprising a discriminator for converting waves of variable frequency into waves of variable amplitude, said discriminator being adapted to pass the maximum frequency deviation of the frequency modulated wave applied to its input, a detector for detecting the wave output of the discriminator into signal frequency currents varying in amplitude in accordance with the frequency deviations of the received radio wave, and 4a. signal frequency correcting circuit con- 'nected to the output circuit of the detector to correct the eil'ects of the unequal frequency deviations in the received waves at different signailing frequencies, said signal frequency correcting network having a pair of input terminals and a pair of output terminals, a condenser and a resistance connected in series to and between one of said input terminals and one of said output terminals, a series circuit connected in shunt to said resistance, said series circuit consisting of a coil vand condenser connected in series, and a resistance connected in shunt to said output terminals.

CLARENCE W. HANSELL.

REFERENCES CITED The following references are of record in the file of this' patent:

UNITED STATES PATENTS Number Name Date 1,794,932 Usselman Mar. 3, 1931 1,871,986 Hamilton Aug. 16, 1932 2,071,113 Crosby Feb. 16, 1937 2,121,150 Jarvis -..i June 21, 1938 2,137,833 Stocker Nov. 22, 1938 2,179,132 Hansell Nov. 7, 1939 

